Estonia - President Obama said Wednesday that the United States will not be intimidated by Islamic State militants after the beheading of a second American journalist and will build a coalition to "degrade and destroy" the group.

Obama still did not give a timeline for deciding on a strategy to go after the extremist group's operations in Syria. "It's going to take time for us to be able to roll them back," the president said at a news conference during a visit to Europe.

The president's comments came after he said the United States had verified the authenticity of a video released Tuesday showing the beheading of freelance reporter Steven Sotloff, two weeks after journalist James Foley was similarly killed.

Obama vowed the U.S. would not forget the "terrible crime against these two fine young men."

"We will not be intimidated. Their horrific acts only unite us as a country and stiffen our resolve to take the fight against these terrorists," Obama said. "And those who make the mistake of harming Americans will learn that we will not forget, and that our reach is long and that justice will be served."

During an appearance in Maine, Vice President Joe Biden said the U.S. would follow the militants to "the gates of hell."

"As a nation, we're united. And when people harm Americans, we don't retreat, we don't' forget," Biden said. "We take care of those who're grieving and when that's finished they should know we will follow them to the gates of hell until they are brought to justice. Because hell is where they'll reside."

Obama also sought to clean up the damage from his statement last week that "we don't have a strategy yet" for dealing with the Islamic State group in Syria. Republicans quickly seized on the remark to argue the president lacks a coherent approach to fighting the extremist group.

"It is very important from my perspective that when we send our pilots in to do a job, that we know that this is a mission that's going to work, that we're very clear on what our objectives are, what our targets are," Obama said. "We've made the case to Congress and we've made the case to the American people, and we've got allies behind us so that it's not just a one-off, but it's something that over time is going to be effective."

In the Sotloff video, a masked militant warns Obama that as long as U.S. air strikes against the Islamic State group continue, "our knife will continue to strike the necks of your people."

Obama responded that the air strikes have been effective in blunting the militant threat and he will continue to battle the "barbaric and ultimately empty vision" that the Islamic State represents. He said he will consult with NATO allies at a summit in Wales Thursday and Friday on a strategy to combat the Islamic State and other militant networks that arise.

"Our objective is clear, and that is to degrade and destroy ISIL so that it's no longer a threat - not just to Iraq, but also the region and to the United States," he said, using an acronym for the militant group.